1. Technical Field
The present invention relates to a phase change random access memory (PRAM), and more particularly, to a phase change random access memory and a method for manufacturing the same.
2. Related Art
In general, a phase change random access memory (PRAM) stores data using a difference in resistance between an amorphous state and a crystalline state, by applying an electrical pulse to a phase change layer formed of a chalcogenide compound. The amorphous state of the phase change layer is obtained by applying a high current to increase the temperature of a phase change substance over a melting point and subsequently performing instant-cooling. The crystalline state of the phase change layer is obtained by generating a nucleus from applying low current in the amorphous state and undergoing a growing process.
Here, in the phase change random access memory, while change from the amorphous state into the crystalline state is easily implemented, change from the crystalline state into the amorphous state consumes large operating current.
In order to reduce operating current and improve reliability in the phase change random access memory, a method for forming a phase change layer as a confined structure may be used.
In forming the confined structure, a phase change area in the form of a hole is defined through a damascene process instead of etching, and a phase change substance is filled in the phase change area to increase phase change efficiency.
In using the confined structure, if phase change substances have uniform composition ratios in the hole, operating current increases since a phase change area is wide. In general, the phase change layer is filled through chemical vapor deposition (CVD) or atomic layer deposition (ALD). In this regard, since a process is performed at a relatively high temperature (around 300° C.), a phase change area is widened due to interatomic diffusion and chemical reaction, and operating current increases.
Also, as the degree of integration of a phase change random access memory increases, the distance between cells decreases and disturbance is caused due to heat so that the reliability of the phase change random access memory deteriorates.